A diesel engine using compression-ignition combustion has advantages such as a high expansion ratio, but has disadvantages such as vibrations, emissions of pollutants including nitrogen oxide (NOx) and dust or the like. Thus, in order to meet increasingly stringent exhaust gas regulation, an exhaust gas after-treatment device including a selective catalytic reduction (SCR) device may be provided.
A reducing agent injection module may be installed in front of the selective catalytic reduction device to inject a reducing agent such as urea toward a flow direction of an exhaust gas. The selective catalytic reduction device may react catalytically the reducing agent with nitrogen oxide (NOx) of the exhaust gas such that the nitrogen oxide is reduced into a nitrogen gas and water. Here, as the reducing agent injection module is exposed to the exhaust gas of high temperature, an engine coolant may circulate through a coolant circulation line connected to the reducing agent injection module to prevent thermal damages on the reducing agent injection module.
However, as an engine stops, a coolant pump may stop to operate so that the flow of the coolant through the coolant circulation line is stopped. In this time, the reducing agent injection module may continue to be heated by the exhaust gas of high temperature, and thus, the reducing agent injection module may be deformed due to the heat or a nozzle of the reducing agent injection module may be blocked by a solid material vaporized from urea.